Cancer is the number one cause of death in Japan, and with aging, the number of patients suffering from cancer has been increasing year by year. Thus, it has been strongly desired to develop a drug or a treatment method, which is highly effective and highly safe. Conventional chemotherapy, radiotherapy and the like have been problematic in that they cause damage to normal cells, in addition to killing cancer cells, and they cause strong side effects. In order to solve this problem, studies have been intensively conducted regarding molecularly targeted therapy, in which a drug targeting a molecule that is expressed specifically in a cancer cell is designed, and the therapy is then carried out using the drug. Among such molecularly targeted cancer therapeutic agents, antibody agents have attracted considerable attention because they are advantageous in terms of their long half-life and a few side effects. Examples of successfully developed cancer therapeutic agents include a chimeric antibody Rituxan that targets CD20 (Non Patent Literature 1), a humanized antibody Herceptin that targets Her2/neu (Non Patent Literature 2), and a humanized antibody Avastin that targets a vascular endothelial growth factor (VEGF). These antibodies have been used for cancer as a target disease, and their therapeutic effects have been recognized.
Antibodies used as therapeutic agents are divided into non-labeled antibodies and labeled antibodies. The action mechanisms of such non-labeled antibodies are considered to be: (1) antibody-dependent cellular cytotoxicity (ADCC) (Non Patent Literature 3) or complement-dependent cytotoxicity (CDC) (Non Patent Literature 4), which are associated with immunocytes or molecules; (2) inhibition of signals associated with intracellular survival or growth by target molecules; (3) induction of apoptosis; and (4) regulation of secretion of cytokines. By combining these mechanisms, the non-labeled antibody kills tumor cells or terminates the growth thereof, so as to exhibit its therapeutic effects. On the other hand, a labeled antibody is formed by linking a radioactive substance or a cytotoxic substance such as a toxin, an enzyme or a drug to an antibody, and the specificity of the antibody is utilized to deliver such a substance only to cancer tissues, so as to achieve the improvement of therapeutic effects and reduction in side effects.
At first, a transferrin receptor (TfR) was identified as a receptor that is present on a reticulocyte as a cell membrane structure for incorporating transferrin (Tf)-bound iron into a cell (Non Patent Literature 5). Thereafter, it was discovered that the transferrin receptor (TfR) is expressed in placental trophoblasts (Non Patent Literatures 10 to 12), in activated lymphocytes (Non Patent Literature 12), and further, in various tumor cells. It has been reported that the transferrin receptor (TfR) is expressed at a high level, for example, in breast cancer (Non Patent Literature 6), prostate cancer (Non Patent Literature 7), lung cancer (Non Patent Literature 8), pancreatic cancer (Non Patent Literature 9), colon cancer (Non Patent Literatures 30 and 31), stomach cancer (Non Patent Literature 31), bladder cancer (Non Patent Literatures 32 and 33), hepatic cancer (Non Patent Literature 34), cervical cancer (Non Patent Literature 35), brain tumor (Non Patent Literature 36), chronic lymphocytic leukemia (Non Patent Literatures 37 and 38), non-Hodgkin's lymphoma (Non Patent Literatures 38 and 39), and adult T-cell leukemia (Non Patent Literature 40). Moreover, since TfR is expressed on the surface of various types of cancer cells at a high level and is expressed in normal cells at a low level, this receptor had been recognized as a molecular target for cancer therapy from long ago (Non Patent Literatures 13 to 16, and Patent Literatures 1 and 2). However, previously developed anti-human TfR antibodies were all derived from animals. It has been generally known that when an antibody derived from an animal other than a human, such as a mouse antibody, is administered to a human, the administered antibody is recognized as a foreign matter, so that a human antibody against the mouse antibody (Human Anti Mouse Antibody: hereinafter referred to as HAMA) is induced in the human body. It has been known that the HAMA reacts with the administered mouse antibody, and causes side effects (Non Patent Literatures 17 to 20) or accelerates the disappearance of the administered mouse antibody from the body (Non Patent Literatures 18, 21 and 22), thereby reducing the therapeutic effects of the mouse antibody (Non Patent Literatures 23 and 24). In fact, a phase 1 clinical testing was carried out using a certain mouse anti-human TfR antibody. As a result, generation of HAMA was observed, and significant therapeutic effects were not found (Non Patent Literature 25).
In order to avoid such a problem, a chimeric antibody was developed (Patent Literatures 3 and 4). The chimeric antibody comprises portions of two or more species-derived antibodies (a mouse antibody variable region and a human antibody constant region, etc.). Such a chimeric antibody is advantageous in that while it retains the characteristics of a mouse antibody, it has human Fc and thus it is able to stimulate a human complement or cytotoxicity. However, such a chimeric antibody still provokes a “human anti-chimeric antibody,” namely HACA (Human Anti-Chimera Antibody) response (Non Patent Literature 26). Moreover, a recombinant antibody, in which only a portion of a substituted antibody is a complementarity determining region (that is, “CDR”) was developed (Patent Literatures 5 and 6). Using a CDR transplantation technique, an antibody consisting of a mouse CDR, and human variable region framework and constant region, namely, “humanized antibody” was produced (Non Patent Literature 27). However, even such a humanized antibody has immunogenicity to humans, and thus, causes a HAHA (Human Anti-Human Antibody) reaction (Non Patent Literatures 28 and 29). Accordingly, it has been desired to develop a more safe and effective antibody therapeutic drug having no immunogenicity, which can be applied to clinical sites.
By the way, it is considered essential for development of antibody drugs to obtain an antibody that recognizes a target cancer antigen “in an intact state” that is present on the surface of a cell membrane. However, it has been difficult to obtain even antibodies against known cancer antigens because the target cancer antigens are membrane proteins. In order to overcome such a problem, the present inventors had produced so far an enormous human antibody library consisting of a hundred billion independent clones, and had established a comprehensive method for obtaining antibodies against proteins existing on the cell membrane surface of cancer cells or tissues (cell surface antigens), utilizing the aforementioned library (Patent Literatures 7 to 9).